The present invention relates to a device for monitoring and measuring isolation, more commonly called "isolation monitor", for an electrical power supply system, such as a single- or three-phase A.C. electrical power supply system, this electrical power supply system being of the isolated neutral type so that the load impedance or impedances are then connected between a phase and this isolated neutral, and not between this phase and earth.
A state-of-the-art isolation monitor of the type mentioned above is for example described in U.S. Pat. No. 5,101,160.
A device of this type operates in the following manner:
Referring to FIG. 1 representing the former art, a three-phase power supply transformer 1, star-connected with isolated neutral N, supplies A.C. electrical power to the three distribution busbars I,II,III of the power supply system which supplies electrical power to several load impedances Z1,Z2, . . . , Zp, . . . , the latter being respectively connected to the same number of three-phase feeders D1,D2, . . . Dp, . . . , of this main distribution system.
It is assumed that one of the load impedances, for example impedance Zp, has an isolation fault with respect to earth which results in the harmful presence, between at least one of the three phase wires of the corresponding feeder Dp and earth, of a leakage impedance Zf constituted by a leakage resistance Rf in parallel with a leakage capacitance Cf.
To detect and measure this fault, an isolation monitor C.I., which is serially connected between the isolated neutral N and earth with a measuring resistance Rm, inputs to the power supply system I,II,III, an A.C. voltage called "reference voltage", of voltage Ui and of a frequency lower than that of the A.C. supply current, this input frequency being for example comprised between 4 and 10 Hertz for a 50 or 60 Hertz A.C. power supply system.
The presence of the isolation fault results, following input of the reference voltage Ui to the power system, in a leakage current If flowing in the leakage impedance Zf, this current If naturally being at the frequency of the input voltage Ui and being fed back to the isolation monitor C.I. via earth and the measuring resistance Rm.
The isolation monitor C.I. in addition itself generates an auxiliary sine wave and cosine wave which enable it, by a conventional process called synchronous demodulation or detection, to determine two components, taken along the same real and imaginary reference axes of the vectors representative of the input voltage Ui and the leakage current If (i.e. in fact the input current). From these components it deduces the phase difference between these vectors Ui and If, and thence the values of the leakage resistance Rf and leakage capacitance Cf.
These state-of-the-art isolation monitors enable an isolation fault occurring somewhere on the power supply system to be detected and measured. They do not however enable this fault to be located, which then means that long and costly fault location is necessary. Furthermore, they are naturally unable to determine whether a single isolation fault at a specific point of the power system is involved, or whether on the other hand multiple isolation faults are involved.